Music to the Ears of a Volcanologist

Popocatepetl, an andesitic volcano located in the Trans-Mexican Volcanic Belt approximately 70 km southeast of Mexico City, Mexico, has been in near-continuous eruption since 1994 and recently experienced a period of higher-than-normal activity beginning in April 2012.

Can automated sound-recognition techniques be used to read and analyze the sounds our planet makes, similar to listening to an orchestra warming up before a concert? DTM’s Diana Roman thinks so.

Much research has been done on understanding a seismic signal known as harmonic tremor, produced by volcanoes, sinkholes, and in hydrothermal and glacial environments. Roman has tuned her ear towards Popocatepetl, an active andesitic volcano located in the Trans-Mexican Volcanic Belt, which has been producing harmonic tremor since 1994.

Roman developed an algorithm for automated detection and characterization of harmonic tremor in continuous seismic data. Such a tool could help us understand the relationship of harmonic tremor to eruptive activity and its value as an eruption precursor.

Details of Roman’s pitch-detection-based algorithm to automatically identify occurrences of harmonic tremor and characterize their frequency content can be found in the 28 June 2017 issue of Geophysical Research Letters.

Episodes of harmonic tremor detected by the automated algorithm versus explosive activity of Popocatepetl during December 2014 to May 2015. The red vertical lines in Figures 3a–3c indicate the timing of all notices of activity of Popocatepetl issued by the Washington Volcano Ash Advisory Center (VAAC) during the period, and black inverted triangles in Figures 3a and 3c show the timing of significant episodes of volcanic activity during the period (see text for details). Figure 3a shows cumulative minutes of detected harmonic tremor through time (data shown by blue dashed line are from CENAPRED station PPP, and data shown by green solid line are from POTREX station ZACA. Figure 3b shows the frequency of the fundamental and first two harmonics for all detected episodes of harmonic tremor (i.e., each set of three vertically aligned dots represents 1 min of detected strong harmonic tremor). Figure 3c shows the calculated HSI of the fundamental frequency (F0) for each minute of detected harmonic tremor. The blue symbols correspond to detections on PPP, and the green symbols correspond to detections on ZACA.